The advantages of welding together structural components, as opposed to other forms of connection such as bolting, are generally known. One type of welding process is known as friction stir welding (FSW). FSW is a solid-state joining process (i.e., the metal is not melted) that uses heat generated between a tool the components to be joined to soften and mechanically intermix portions of the component. The softened material is then joined using mechanical pressure, which may be applied by the tool. While FSW, as compared to other forms of welding, avoids problems associated with cooling material from the liquid phase, there is a general concern as to the sufficiency of the weld formed by FSW due to tunnel defects, discontinuities in the bond due to lack of forging, and incomplete penetration of the tool into the components.